FIG. 1 shows the carriage driving mechanism of a general serial printer The revolutions of the DC motor 1 called a carriage motor is converted into a linear motion of the timing belt 5 via the pulley 3. While by driving with the timing belt 5, the carriage 9 mounted with a printing head 7 is moved, printing is carried out with the printing head 7 on the recording media such as paper.
Hitherto, to control the revolution speed of the carriage motor 1, PI control shown on FIG. 2 has been generally used. This control employs a compensator 19 where the proportion element 15 having a gain K1 and the integration element 17 having a gain K2 are combined in parallel. On receipt of the difference between the revolution speed of the DC motor 1 as detected by the rotary encoder 13 and the standard speed generated from the standard speed signal generator 21, the compensator 19 controls the output of the motor 1 so as to bring its revolution speed in line with the standard speed.
In general, to ensure a good printing quality with the serial printer, the printing is performed as shown in FIG. 3 , after the revolution speed of the carriage motor 1 has regulated to the standard speed. In this case, the travel distance D1 required to regulate the revolution speed of the carriage motor 1 to the standard speed is desired to be as small as possible in order to realize a small-sized serial printer. Consequently, the revolution speed of the carriage motor 1 is required to rise to the standard speed in the shortest time possible and yet to be regulated stably with little overshoots or undershoots.
In the conventional control device, however, the stability and response are determined by the characteristic of the control system composed of the compensator, DC motor and speed detector. Furthermore, in general, the stability and response are contradictory to each other, and consequently, for example, giving priority to the stability worsens the response, thus there is a limitation in satisfying the foregoing requirements.
Furthermore, to yield a good printing quality and a smooth carriage run, it is necessary to control the carriage motor to the standard revolution speed with a high precision. In the conventional serial printer, the revolution speed of the carriage motor (DC motor) is controlled by driving it with a pulse-shaped driving voltage, and the PWM (Pulse Width Modulation) control is employed as a typical method. FIG. 4 is a block diagram showing the conventional control. The unit 23 outputs the standard revolution speed signal and the unit 25 generates a duty signal having the duty corresponding to the standard revolution speed. By performing ON/OFF in response to the duty signal, the driving circuit 29 converts the voltage from the DC power source 27 into a pulse-shaped driving voltage for supply to the carriage motor 1. The unit 25, as shown in FIG. 5(A), determines for each PWM standard period T.sub.WIDTH, the value of duty T.sub.DUTY[n] in each period, and outputs the duty signal. The value of the duty T.sub.DUTY[n] is generally determined according to the difference between the revolution speed which is detected by the rotary encoder fitted to the shaft of the carriage motor 1 and the standard revolution speed.
For simplicity of the control process and so forth, in general, the value of the PWM standard period T.sub.WIDTH is constant. On the other hand, to make the revolution speed agree with the predetermined standard revolution speed, the duty T.sub.DUTY[n] varies. When the duty T.sub.DUTY[n] varies as in FIG. 5, the current shown in FIG. 5 flows in the carriage motor 1. Since the torque produced by the DC motor is proportional to the magnitude of the current, such a variation of current causes a periodic pulsation of torque. Consequently, the revolution speed agrees with the standard revolution speed on an average. In actual cases, however, as shown in FIG. 6, it varies over and below the standard revolution speed while generating a minute oscillation of the period identical to the PWM standard period. The majority of serial printers have plural speed modes according to the printing density With increase of the printing density, the standard rotation speed becomes lower. In general, the lower the standard revolution speed, the smaller the torque demanded of the carriage motor 1, i.e., the value of the current flowing in the carriage motor 1. However, at a smaller standard revolution speed (though the duty T.sub.DUTY becomes small), the counter cmf of the carriage motor 1 is small, and as a result the current peak value is liable to become large, magnifying the periodic pulsation of the torque. When the standard revolution speed is small as above, despite the small torque as required, a torque more than necessary is produced because of the large current peak value. Such a larger torque than necessary is desired to be eliminated.
The periodic minute oscillation due to the variation of torque of the carriage motor 1 generates a noise. In a frequency analysis of this noise a large noise peak appears at the PWM standard frequency f.sub.WIDTH (=1/T.sub.WIDTH) as shown in FIG. 7. Since the value of the PWM standard frequency f.sub.WIDTH falls in the frequency band of several kHz to several ten kHz, this noise is undesirable on the hearing sense.